The present invention relates to a resin mold-type solid electrolytic capacitor using niobium or an alloy containing niobium as the main component as an anode member.
Nowadays, as an anode body of an electrolytic capacitor, a winding of surface-roughened aluminum foil, a single layer body or multilayer body of a surface-roughened aluminum thin plate, a porous sintered body of tantalum powder and the like are frequently used. As material of the anode body, niobium also comes under the spotlight. Niobium is a metal belonging to the same group, i.e., 5A, as tantalum in the periodic table of the elements and having physical properties similar to those of tantalum, has many advantages such as a smaller specific gravity, a larger amount of reserves in the earth crust and a lower kg unit cost compared with tantalum. Therefore, trials to utilize niobium as an anode member of an electrolytic capacitor have been done. However, there were many problems in the case of an electrolytic capacitor using niobium as an anode member that a leakage current (LC) tended to become large, an aging treatment (an operation for insulating a defective portion of a dielectric oxide film by applying a direct-current voltage according to the polarity of a capacitor for a long period of time) for reducing the leakage current did not achieve easily a desired effect, and a value of capacitance was apt to vary depending on a direct-bias voltage, which problems were impossible to be solved by converting simply the technique employed when tantalum was used as the anode body. These problems have been gradually overcome through many studies and improvements. However, the final production step, that is, a forming (hereinafter, referred to as molding) step is also faced with a large problem.
Nowadays, many capacitors using a tantalum sintered body as an anode body are produced. The capacitor is constructed by forming a dielectric coating film, a solid state electrolyte layer, and cathode lead-out layer in order on the surface of the anode body made of the tantalum sintered body, connecting an anode lead member implanted to an end of the anode body with an anode terminal, connecting the cathode lead-out layer with a cathode terminal, and coating and sealing by a sheath resin.
Here, at the step of forming the sheath resin, the sheath resin is formed by so called injection molding, in which the capacitor is set in a sheath mold, solid epoxy resin and the like is molten, for example, by heating at about 180° C., the molted resin is pressed into a cavity provided concavely in the mold with a high pressure of around 100 atm and the temperature is kept for several minutes to cure thermally. Then, it is taken out of the mold, subjected to postcure when required, and subjected to a well-known aging by applying voltage to be completed. The injection molding has such an advantage that it allows to produce easily a molded part having a high dimensional accuracy.
However, since an injection pressure at injection molding is large, the dielectric coating film on the surface of the anode body is injured due to a mechanical damage at the injection molding to result in an increase in the leakage current supposedly caused by this.
As a conventional technique to solve the problem, a method is proposed, in which a cushioning material formed of synthetic fiber, rubber, paper, cloth or the like is provided only on the opposite face of a gate of the mold by injection molding to buffer an injection pressure of the molten resin. (For example, refer to JP-A-8-148392)
However, when a capacitor element using an anode body with niobium as the main component is molded by the same method as that for the tantalum element, the finished capacitor often has been heavily deteriorated, can not be repaired sufficiently by being subjected to a well known aging by applying a voltage at high temperatures and decreased in the leakage current to a practical level. Further, even when a capacitor having satisfactory initial properties can be obtained by applying less severe conditions during molding, the capacitor element receives a destructive damage by a solder heat resistance test (EIAJ RC-2378, corresponding to 250° C.), which can not be repaired even when the aging is conducted.
The cause is attributed to a low mechanical and thermal strength of the niobium element. That is, in addition to a mechanical damage caused by violent collision of a heat-molten resin having an extremely high viscosity with a high pressure of around 100 atm to the niobium sintered body having a slightly inferior mechanical strength because of its melting point lower than that of tantalum by 520° C., there is such a weak point peculiar to niobium that a oxidized niobium film being a dielectric coating film generates disturbance of atomic level at high temperatures of the solder heat resistance test.
Further, instead of a conventional lead solder, a lead-free solder, which gives a less adverse affect to the environment, has been used. However, since the lead-free solder has a higher melting point compared with the lead solder, there is a problem that the capacitor properties deteriorate much when a capacitor using an anode body containing niobium as the main component is subjected to soldering by using the lead-free solder.